[sandbox] fix read for buffers of zero length

This patch fixes the read function for the linux sandbox if a buffer
of zero length should be read. The error handling introduced in
6574529bb5 detects a false positive if the
length is zero.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

arch/sandbox/lib/common.c

@@ -148,6 +148,9 @@ int linux_read(int fd, void *buf, size_t count)
 {
 	ssize_t ret;
 
+	if (count == 0)
+		return 0;
+
 	do {
 		ret = read(fd, buf, count);